Siegbahn was educated at the University of Lund and obtained his doctorate there in 1911. At Lund he became a research assistant to Johannes Rydberg and succeeded Rydberg as professor of physics in 1920. In 1916 Siegbahn discovered a new group of wavelengths, the M series, in X-ray emission spectra. He developed equipment and techniques that allowed him and subsequent researchers to determine accurately the wavelengths of X rays. A year after he became professor of physics at Uppsala University, he and his colleagues furnished proof (1924) that X rays are refracted (bent) when they pass through prisms, just as light rays are, though the effect is weaker and obscured by absorption of the X rays. Later, Siegbahn also investigated the weaker X rays that lie near the ultraviolet region of the spectrum.

In 1937 Siegbahn became professor of physics at the University of Stockholm. In the same year the Swedish Royal Academy of Sciences created the Nobel Institute of Physics at Stockholm and appointed Siegbahn its director; he retired from that position in 1975. He served as a member of the International Committee on Weights and Measures from 1939 until 1964. His son, Kai Manne Börje Siegbahn, also became a physicist and won the Nobel Prize for Physics in 1981.